This invention relates to a sheet transport apparatus for transporting a sheet along a predetermined path and in particular to improvements in a sheet transport apparatus used for changing the sheet transport direction and an image formation apparatus using the sheet transport apparatus.
Generally, an image formation apparatus using electrophotography, etc., adopts, for example, a technique of forming an electrostatic latent image responsive to an image signal on a latent image support such as a photoconductor drum and transferring an image provided by developing the electrostatic latent image (toner image) directly or through an intermediate transfer body to a sheet of paper, etc.
In such an image formation apparatus, to precisely transfer an image onto a sheet, usually the sheet is registered to a predetermined reference position, and as a registering technique, side registration of registering, for example, a side of a sheet to a predetermined side reference position and then sending the sheet to a transfer position is available.
In this kind of side registration, usually a skew correction of correcting the transport attitude of a sheet is performed.
The skew correction is as follows: For example, a skew roll as an aligner is disposed in a sheet transport passage, a side guide is disposed on one side of the sheet transport passage, and the sheet transported by the skew roll is pressed against the side guide, whereby the side of the sheet is registered to a reference position provided by the side guide and the transport attitude of the sheet is corrected.
As lead registration of registering the lead of a sheet, a roll nip technique of once stopping a sheet in a nip area of a registration roll and registering the lead of the sheet at the restart timing of the registration roll, a gate technique of once stopping a sheet at a gate and registering the lead of the sheet at the opening timing of the gate, a roll rotation control technique of controlling the number of rotations, etc., of a registration roll for making the transport speed of a sheet variable and adjusting the arrival timing of the sheet at a transfer position, or the like is widely adopted.
In such a skew correction technique in the related art (skew roll+side guide), the following technical problems are found:
First, the types of sheets that can be used are limited drastically.
That is, in this kind of skew correction, if the sheet type (for example, paper thickness, rigidity, material property of pulp, presence or absence of coat layer, thickness of coat layer, material property of coat layer, etc.) differs, the transport force, etc., of the sheet changes and variations in skew or the arrival time of the sheet at the target position change for the worse.
For example, if the thickness of a sheet changes, the sheet rigidity and the transport resistance between the sheet and a chute change. If the coat material of a surface coat layer changes, the coat layer adheres to the skew roll the frictional resistance between the skew roll and the sheet changes, and variations in the transport force become large. In addition, if the environment changes, further the variations in the transport force change for the worse.
Thus, in the skew correction technique in the related art, the types of sheets that can be used are limited to a considerably narrow range (for example, basis weight in the range of 65 to 95 gsm, non-coated paper).
Second, if the skew correction technique using “skew roll and side guide” is adopted, slip transport is executed and thus the variations in the arrival time of the sheet at the target position become large.
Generally, most of sheet transport is grip transport wherein a drive transport roll and a driven transport roll rotate in the same direction and a sheet is always transported with the sheet gripped.
In the grip transport, a slip scarcely occurs between the sheet and the transport roll and a transport speed error is caused by roll diameter variations (tolerance) and drive system variations; the value is small and particularly the tolerance is lessened, whereby the error can be easily minimized.
Since a plurality of transport rolls are placed axially and the sheet is supported on several transport rolls in the transport direction, the transport error becomes an average value thereof and thus the diameter variations are largely decreased and the variations in the arrival time of the sheet are also lessened (for example, ±30 ms or less).
However, if the skew correction technique using “skew roll and side guide” is adopted, a drive transport roll and a driven transport roll rotate in different directions and a sheet is always transported in a slip state.
In this case, the sheet is transported by an unstable transport force of slip between the sheet and the transport roll and thus it is impossible to precisely control registration of the lead of the sheet and consequently the variations in the arrival time of the sheet at the target position become large.
More specifically, to perform registration of the lead of the sheet with high accuracy, after skew correction of the sheet is made, often the roll rotation control technique of controlling the number of rotations, etc., of a registration roll for making the transport speed of a sheet variable and adjusting the arrival timing of the sheet at a transfer position is adopted.
Under such circumstances, the adjustment range of the arrival timing of the sheet at the transfer position is determined by the process speed and the pitch between sheets; a system with a larger adjustment range value is resistant to disturbance.
Here, to enlarge the adjustment range, (1) the process speed must be made lower or (2) the pitch between sheets must be enlarged; normally, to raise the productivity of an image formation apparatus as much as possible, the process speed must be increased as much as possible and the pitch between sheets must be lessened as much as possible. It is not easy to enlarge the adjustment range sufficiently.
On the other hand, to execute skew correction, if slip transport technique is adopted as a skew roll, the variations in the arrival time of the sheet become large (for example, 200 ms or more) and may be placed out of the adjustment range under the rotation speed control of downstream registration roll and may be placed out of control. Particularly, if an attempt is made to transport various sheets or the environment changes, the variations become noticeably large.
Third, at the skew correction time with the skew roll, a stress is placed on a sheet.
That is, in the skew correction in the related art, the inclination of the skew roll is fixed uniquely and thus a force attempting to advance in a slanting direction occurs on the lead of a sheet at the same time as the sheet enters the nip area of the skew roll, but a force attempting to advance straightly continues to occur on the tail of the sheet and a stress is given to the sheet accordingly.
Since the inclination of the skew roll is fixed uniquely, the force of the component in a direction at right angles to the transport direction (skew correction force) continues to occur on the sheet after undergoing the skew correction.
Thus, the frictional resistance force provided by multiplying a friction coefficient by the skew correction force occurs between the sheet edge and side guide and becomes a resistance force to sheet transport, giving a stress to the sheet.
Particularly, if high nip pressure is required for skew correction as with a cardboard, the frictional resistance force becomes large.
Fourthly, if the variations in the arrival time of a sheet at a transfer position are suppressed, skew worsens because of secondary trouble.
That is, to suppress the variations in the arrival time of the sheet, if the nip pressure of the skew roll is strengthened for increasing the transport force, it is made possible to stabilize the transport time of the sheet.
However, if the strength of the nip pressure of the skew roll is not adequate, the transport force may be too increased and the frictional resistance force between the sheet and side guide may become large, placing the sheet in a stop state.
However, the force in the transport direction and the force in the side guide direction (skew correction force) occur on the skew roll and thus if the nip pressure of the skew roll is strengthened, the skew correction force of pressing the sheet against the side guide is also increased inevitably. If the skew correction force is increased, it is feared that the sheet may buckle and skew may worsen, namely, secondary trouble may occur.
Thus, strengthening the nip pressure also involves an adequate range and the skew correction force cannot be ignored and thus it is not easy to strengthen the nip pressure for stabilizing the arrival time of the sheet.